All of the following are modern polling methods except:

a. pollster interviews
b. mail questionnaires
c. in-person interviews
d. telephone interviews

I can't find this answer anywhere in my textbook. Help!

Mail in are self-selecting. Wasn't that one of the errors in the Truman-Dewey race in 1948?

This explains how samples are selected for polling. Yes, mail-in (and phone-in) samples are self-selecting and not representative of the larger population.

https://www.aapor.org/Education-Resources/Election-Polling-Resources/Sampling-Methods-for-Political-Polling.aspx

I thought it doesn't matter if they are self-selecting or not for this question

Can someone help me please I don't understand

You can't have a self-selecting poll for it to be representative.

What is the difference between a pollster interview and an in-person interview?

I tried to find out online. What does your text say?

It doesn't mention pollster interviews at all.

I vote for b.

I would pick B because it is not modern at all and I do not think it is even used seriously any more.